I'm wondering if anybody has noticed any wear on the firebox? Fire eats away mild steel. That's why most combustion chambers in stoves etc are lined with firebricks.
The reason I ask is that firebox of the mini or any GF for that matter is not very repairable. The whole firebox is welded to the frame, inner and outer sheeting.
A lot of outdoor furnaces are made of CorTen. They seem to hold up pretty well. In the grand scheme of things switching from mild steel to CorTen might be worthwhile if it prolonges the life smoker. Not that I don't like a DIY project but a long lasting heirloom is always nicer.
Perhaps I'm overthinking this but the question about the durability of the firebox in the GF has never asked.

Keep the ash cleaned out and store it indoors (away from moisture, saltwater and humidity) and it will last a really long time. I’ve been here a number of years and don’t recall anyone having a firebox burn thru on a gravity feed.

@Dirtytires, I've reading along also for some years and have the plans already for two years when more urgent stuff came up. Now I'm trying to start this build again. While discussing the build some buddies suggested using CorTen. But is the CorTen(A) really more resistant to fire than regular fire? And if so could you make the entire firebox out 1/4"? 1/2" CorTen does exist but is rare and expensive. Cutting everything out of a single (gauge) sheet is way more cost effective. But I'm unsure if the reduction in thickness works.

Sorry if I misled you. I have absolutely no experience with CorTen. My point was that we have built hundreds of smokers on this site and burn thru on fireboxes does not appear to be a big issue as long as it’s maintained properly. With that said, I’m not sure ANY further enhancements need to be made.

Keep in mind, a furnace can run 24/7/365. They also use different fuels like natural gas which has more moisture in it. It will take me a lifetime to put a Year’s worth of furnace abuse on a smoker.

I have some experience at a local oil fired power plant. They are more concerned with the number of heat cycles than the heat itself.

@Diggerg, free is always nice.
At the shop they don't stock plate over 6mm. Everything above that is ordered at a cutting service. But if I need the order thicker plate I can also order CorTen. My question remains wheter it's more resistant to wear.
At the moment my drawings use 6mm plate as substitute for 1/4" and 130x12 bar for the 1/2" firebox section. (all mild steel)